Adding IOTA Foundation to USERS.MD (#4436)

Summary:
Adding IOTA Foundation to USERS.MD
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4436

Differential Revision: D10108142

Pulled By: sagar0

fbshipit-source-id: 948dc9f7169cec5c113ae347f1af765a41355aae

USERS.md

@@ -89,3 +89,6 @@ LzLabs is using RocksDB as a storage engine in their multi-database distributed
 
 ## ProfaneDB
 [ProfaneDB](https://profanedb.gitlab.io/) is a database for Protocol Buffers, and uses RocksDB for storage. It is accessible via gRPC, and the schema is defined using directly `.proto` files.
+
+## IOTA Foundation
+ [IOTA Foundation](https://www.iota.org/) is using RocksDB in the [IOTA Reference Implementation (IRI)](https://github.com/iotaledger/iri) to store the local state of the Tangle. The Tangle is the first open-source distributed ledger powering the future of the Internet of Things.
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